Dudplication is a feature that uses a DDT (dudplication table) stored in the system cache space (RAM or SSD) to keep mapping records of unique data. Via checking the DDT to verify whether data block being written is duplicated or not, the system then decides if the data can be deduplicated.
By using deduplication, in cases that a lot of duplicated data are stored, user can benefit from reclaiming the space back from deduplicated data. 
Yes, enabling deduplication greatly reduces the zfs performance since with each read and write, the action of checking through the DDT is required. In theory the larger the DDT is, the greater the impact is to the write performance. 
Qsan NAS products use block level deduplication, which means the data is checked block by block and a footprint is generated for each block, then recorded in the DDT along with its address.
Block level deduplication works with higher efficiency then file level deduplication, as the possibility of block level duplication is higher than that of file level. (A file is always consisted of multiple blocks and therefore the complexity at file level is higher than block level, therefore duplication possibility is lower than block level.)
Qsan NAS products use inline deduplication. All data being written are checked and deduplicated before actually written. (Post processing means the data is firstly written and later on deduplicated.) 
All data stored in the zfs prior to enabling deduplication cannot be further deduplicated.
Storing the DDT could be very memory-consuming. It is therefore suggested to always have SSD read cache disks configured for using deduplication feature.
The dedup limit displayed in the dashboard merely calculates how much deduplicated data the system can afford to store in the system RAM cache without SSD cache enabled, and displays how much memory quota (allocated for deduplication usage) has been used.
If the deduplication limit (for system) is exceeded:
1. If the dedupped zfs resides in a pool with SSD read cache, it's not affected.
2. If the dedupped zfs resides in a pool without SSD read cache (where the DDT in this case is stored in the system RAM cache), the dedup feature for this zfs will be automatically disabled and cannot be enabled again. All later-stored data will be stored with standard non-dedupped manner and therefore cause the zfs to become a hybrid instant with both dedupped and non-dedupped data inside.
The maximum number of U1 series local user/groupaccounts iare 4096/512. 
The maximum number of U2 series local user/groupaccounts are 10000/512. 
The maximum number of U3/U4/U6 series local user/group accounts are 10000/1024.